1. Field of the Invention
The invention is generally related to the area of optical data communications, and more particularly, related to add/drop optical devices or subsystems.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. Fiber optic networks are becoming increasingly popular for data transmission due to their high speed, high capacity capabilities. Wavelength division multiplexing (WDM) is a technology that puts data from different sources together on an optical fiber with each signal carried at the same time on its own separate light wavelength. Using the WDM system, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. To take the benefits and advantages offered by the WDM system, there require many sophisticated optical network elements.
A tunable WDM add/drop system is one of such elements that are designed to add or drop one or more specific wavelengths or channel signals. In a fiber optic network, there are cases of exchanging wavelengths among optical signals on “loops” within networks. The exchanges occur at points where two or more loops intersect for the purpose of exchanging wavelengths. In particular, the exchanging of data signals involves the exchanging of matching wavelengths from two different loops within an optical network. In other words, a signal would drop a wavelength to the other loop while simultaneously adding the matching wavelength from the other loop. The adding and dropping of wavelengths often occur together. Each wavelength is commonly referred to as a channel or data channel. A tunable WDM add/drop system exists at the points to facilitate these exchanges.
In general, tunable WDM add/drop systems often utilize fixed or tunable fiber Bragg gratings (FBG) to provide the necessary wavelength selectivity for the add/drop function. To add or drop a specific wavelength, the accurate control of the signal at an absolute wavelength is of high requirement. Any deficient design in the tuning wavelength accuracy could lead to problems that may include optical cross talk, signal fluctuation and numerous other undesirable effects.
There have been many efforts in design absolute wavelength selectivity. One exemplary technology is to fabricate wavelength selective elements based on recording an index of refraction grating in the core of an optical fiber, for instance, disclosed in U.S. Pat. No. 4,474,427 to Hill et al. and U.S. Pat. No. 4,725,110 to Glenn et al. A difficulty with conventional fiber Bragg gratings is that they filter only a fixed wavelength. Each grating selectively reflects light in only a narrow bandwidth centered around a desired wavelength. However, in many applications, such as tunable multiplexing, it is desirable to have a grating whose wavelength response can be tuned, that is, controllably altered for a desired wavelength. A tunable FBG can be realized by stretching, compressing or heating the device. The tunable designs, on the other hand, have stringent requirements of accurate control over an absolute wavelength to ensure the proper functioning of the system. Therefore, there is a need for techniques of how to control effectively and precisely the wavelength selectivity of a tunable device.